The neoplastic change in tissue culture is studied by four different approaches. 1. Serum dependence of neoplastic and non-neoplastic cells and its possible relation to the C-AMP content of the cells. 2. Selective susceptibility of neoplastic cells to BCG activated macrophages. 3. The organization of microtubules and microfilaments in neoplastic and non-neoplastic cells. 4. Growth of non-neoplastic and neoplastic cells in agarose.